Astronomy & Astrophysics manuscript no. Barbarians c ESO 2017 July 21, 2017 Shape and spin determination of Barbarian asteroids M. Devogèle1, 2, P. Tanga2, P. Bendjoya2, J.P. Rivet2, J. Surdej1, J. Hanuš2, 3, L. Abe2, P. Antonini4, R.A. Artola5, M. Audejean4, 7, R. Behrend4, 8, F. Berski9, J.G. Bosch4, M. Bronikowska6, A. Carbognani12, F. Char10, M.-J. Kim11, Y.-J. Choi11, C.A. Colazo5, J. Coloma4, D. Coward13, R. Durkee14, O. Erece15, 16, E. Forne4, P. Hickson17, R. Hirsch9, J. Horbowicz9, K. Kaminski´ 9, P. Kankiewicz18, M. Kaplan15, T. Kwiatkowski9, I. Konstanciak9, A. Kruszewki9,V. Kudak19, 20, F. Manzini4, 21, H.-K. Moon11, A. Marciniak9, M. Murawiecka22, J. Nadolny23, 24, W. Ogłoza25, J.L Ortiz26, D. Oszkiewicz9, H. Pallares4, N. Peixinho10, 27, R. Poncy4, F. Reyes28, J.A. de los Reyes29, T. Santana–Ros9, K. Sobkowiak9, S. Pastor29, F. Pilcher30, M.C. Quiñones5, P. Trela9, and D. Vernet2 (Affiliations can be found after the references) Received . ; accepted . ABSTRACT Context. The so-called Barbarian asteroids share peculiar, but common polarimetric properties, probably related to both their shape and composition. They are named after (234) Barbara, the first on which such properties were identified. As has been suggested, large scale topographic features could play a role in the polarimetric response, if the shapes of Barbarians are particularly irregular and present a variety of scattering/incidence angles. This idea is supported by the shape of (234) Barbara, that appears to be deeply excavated by wide concave areas revealed by photometry and stellar occultations. Aims. With these motivations, we started an observation campaign to characterise the shape and rotation properties of Small Main- Belt Asteroid Spectroscopic Survey (SMASS) type L and Ld asteroids. As many of them show long rotation periods, we activated a worldwide network of observers to obtain a dense temporal coverage. Methods. We used light-curve inversion technique in order to determine the sidereal rotation periods of 15 asteroids and the con- vergence to a stable shape and pole coordinates for 8 of them. By using available data from occultations, we are able to scale some shapes to an absolute size. We also study the rotation periods of our sample looking for confirmation of the suspected abundance of asteroids with long rotation periods. Results. Our results show that the shape models of our sample do not seem to have peculiar properties with respect to asteroids with similar size, while an excess of slow rotators is most probably confirmed. Conclusions. Key words. asteroids – asteroid shape – asteroid rotation 1. Introduction surfaces derivation technique (FSDT) which considers flat sur- faces on such models to obtain indications about the possible Shape modeling is of primary importance in the study of aster- presence of large concavities. oid properties. In the last decades, 1000 asteroids have had a shape model determined by using inversion⇠ techniques1; most of Cellino et al. (2006) reported the discovery of the anomalous them are represented by convex shape models. This is due to the polarimetric behaviour of (234) Barbara. Polarisation measure- fact that the most used technique when inverting the observed ments are commonly used to investigate asteroid surface proper- light-curves of an asteroid – the so-called light-curve inversion – ties and albedos. Usually, the polarisation rate is defined as the is proven to mathematically converge to a unique solution only di↵erence of the photometric intensity in the directions paral- if the convex hypothesis is enforced (Kaasalainen and Torppa lel and perpendicular to the scattering plane (normalised to their I I ?− k 2001; Kaasalainen et al. 2001). At the typical phase angles ob- sum): Pr = I +I . It is well known that the intensity of polarisa- served for Main Belt asteroids, the light-curve is almost insen- tion is related? tok the phase angle, that is, to the angle between sitive to the presence of concavities according to Durechˇ and the light-source direction and the observer, as seen from the ob- Kaasalainen (2003). Although the classical light-curve inver- ject. The morphology of the phase-polarisation curve has some sion process cannot model concavities, Kaasalainen et al. (2001) general properties that are mainly dependent on the albedo of the point out that the convex shape model obtained by the inversion surface. A feature common to all asteroids is a “negative polari- is actually very close to the convex hull of the asteroid shape. sation branch” for small phase angles, corresponding to a higher Since the convex hull corresponds to the minimal envelope that polarisation parallel to the scattering plane. For most asteroids, contains the non-convex shape, the location of concavities cor- the transition to positive polarisation occurs at an “inversion an- responds to flat areas. Devogèle et al. (2015) developed the flat gle” of 20◦. In the case of (234) Barbara, the negative polarisa- tion branch⇠ is much stronger and exhibits an uncommonly large 1 See the Database of Asteroid Models from Inversion Techniques value for the inversion angle of approximately 30◦. Other Bar- (DAMIT) data base for an up-to-date list of asteroid shape models: barians were found later on (Gil-Hutton et al. 2008; Masiero and http://astro.troja.m↵.cuni.cz/projects/asteroids3D/ Cellino 2009; Cellino et al. 2014; Gil-Hutton et al. 2014; Bag- Article number, page 1 of 23 A&A proofs: manuscript no. Barbarians nulo et al. 2015; Devogèle et al. 2017), and very recently, it was of some individual asteroids are presented as well as a general discovered that the family of Watsonia is composed of Barbar- discussion about the main results of the present work. The inci- ians (Cellino et al. 2014). dence of concavities and the distribution of the rotation periods Several hypotheses have been formulated in the past to ex- and pole orientations of L-type asteroids are presented. Finally plain this anomaly. Strong backscattering and single-particle Sect. 5 contains the conclusions of this work. scattering on high-albedo inclusions were invoked. Near- infrared (NIR) spectra exhibit an absorption feature that has been related to the presence of spinel absorption from flu↵y- 2. The observation campaigns type Calcium-Aluminium rich Inclusions (CAIs) (Sunshine et al. 2.1. Instruments and sites 2007, 2008). The meteorite analogue of these asteroids would be similar to CO3/CV3 meteorites, but with a surprisingly high CAI Our photometric campaigns involve 15 di↵erent telescopes and abundance ( 30%), never found in Earth samples. If this inter- sites distributed on a wide range in Earth longitudes in order pretation is confirmed,⇠ the Barbarians should have formed in an to optimise the time coverage of slowly rotating asteroids, with environment very rich in refractory materials, and would con- minimum gaps. This is the only strategy to complete light-curves tain the most ancient mineral assemblages of the Solar System. when the rotation periods are close to 1 day, or longer, for This fact link, the explanation of the polarisation to the pres- ground-based observers. ence of high-albedo CAIs, is the main motivation for the study The participating observatories (Minor Planet Center (MPC) of Barbarians, whose composition challenges our knowledge on code in parentheses) and instruments, ordered according to their meteorites and on the mechanisms of the formation of the early geographic longitudes (also Tab. 1), are: Solar System. – the 0.8m telescope of Observatori Astronòmic del Montsec The compositional link is confirmed by the evidence that all (OAdM), Spain (C65), Barbarians belong to the L and Ld classes of the Small Main- – the 1.04m “Omicron” telescope at the Centre Pédagogique Belt Asteroid Spectroscopic Survey (SMASS) taxonomy (Bus Planète et Univers (C2PU) facility, Calern observatory (Ob- and Binzel 2002), with a few exceptions of the K class. While servatoire de la Côte d’Azur), France (010), not all of them have a near-infrared spectrum, all Barbarians are – the 0.81m telescope from the Astronomical Observatory of found to be L-type in the DeMeo et al. (2009) NIR-inclusive the Autonomous Region of the Aosta Valley (OAVdA), Italy classification (Devogèle et al. 2017). (B04), However, the variety of polarimetric and spectroscopic prop- – the 0.4m telescope of the Borowiec Observatory, Poland erties among the known Barbarians, also suggests that composi- (187), tion cannot be the only reason for the anomalous scattering prop- – the 0.6m telescope from the Mt. Suhora Observatory, Cra- erties. In particular, Cellino et al. (2006) suggested the possible cow, Poland, presence of large concavities on the asteroid surfaces, resulting – the 0.35m telescope from Jan Kochanowski University in in a variety of (large) scattering and incidence angles. This could Kielce, Poland (B02), have a non-negligible influence on the detected polarisation, but – the 1m Zadko telescope near Perth, Australia (D20), such a possibility has remained without an observational verifi- – the 0.61m telescope of the Sobaeksan Optical Astronomy cation up to now. Observatory (SOAO), South Korea (345), Recent results concerning the prototype of the class, – the 0.7m telescope of Winer Observatory (RBT), Arizona, (234) Barbara, show the presence of large-scale concavities USA (648), spanning a significant fraction of the object size. In particu- – the 0.35m telescope from the Organ Mesa Observaory lar, interferometric measurements (Delbo et al. 2009) and well- (OMO), NM, USA (G50), sampled profiles of the asteroid obtained during two stellar oc- – the 0.6m telescope of the Southern Association for Re- cultations indicate the presence of large-scale concave features search in Astrophysics (SARA), La Serena Observatory, (Tanga et al. 2015). Testing the shape of other Barbarians for the Chile (807), presence of concavities is one of the main goals of the present – the 0.35m telescope from the UBC Southern Observatory, La study, summarising the results of several years of observation.